On Southern Hyogo-Prefecture Earthquake and Some Related Activities in Japan
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XA9952646 SMiRTl3 IAEA-WS August 1995 Iguasu, Argentin On Southern Hyogo-prefecture Earthquake and Some Related Activities in Japan Heki SHIBATA, Professor-Dr. Mechanical Eng'g. and Materials Sci. Faculty of Engineering Yokohama National University 156 Tokiwadai, Hodogaya, Yokohama 240, JAPAN Fax 81/45-331-6593 Abstract This paper consists of three parts. At first the reporter discusses on the earthquake event on January 17, 1995, and then on the summary of the report of examining the adequecy of the guideline of seismic design of nuclear power plants in Japan by the task group, Nuclear Safety Commission. And also on the activity of "the sub-committee on the research of seismic safety" for the future research subjects during 1996 ~ 2000 F.Y. Part 1 On the Southern Hyogo-prefecture Earthquake 1.1 Introduction The event occured at 5:46 am on January 17, 1995 was one of the most serious earthquake disasters since the Kwanto earthquake in 1923 in Japan. In the Kwanto earthquake, approximately 140,000 were killed in Tokyo and Yokohama area. Most of casualties were caused by the extended fires in Cities of Tokyo and Yokohama. The direct deaths are estimated as 10,000. In the earthquake disaster of this time, officially called as "Hanshin-Awaji Great Earthquake Disaster", approximately 5,500 were killed, and those were directly caused by structural failure mostly. One of the serious natural disasters, which caused such a big number of deaths is "Isewan Typhoon and High-tide" in 1959. 37 As seismic events, this earthquake is one of the greatest ones, may be once or twice every century through Japan. In this area, we experienced several destructive seismic events since 8 Q. but they might be not so serious ones according to historical records including 16 9 one: which is famous. Some one estimated its return period might be more than 5,000 years (in Fig.8). The characteristics of its ground motion was very unique, that is, very high velocity as well as high acceleration and short duration (Table 1). This caused many casualties and serious, very uneque structural failures. The feature of the ground motion, damage of various kind of structures, and other unique events will be described in the following chapters, which were presented at SMiRT 13 in Porto Alegre last August. Of course, this area is one of the most densely populated and highly industrialized areas. The western part of Kei-han-shin area, which is the second busiest area in Japan, is centered by City of Kobe. This is the reason why this event is called as Kobe earthquake. Officialy. Southern Hyogo-prefecture earthquake 1995 and the disaster itself is called as Hanshin-Awaji Great Earthquake Disaster. 1.2 Purpose of Field Survey What are the purposes of the survey on seismic damages? How do we learn the lessons from the seismic event. Usually we start immeadately after the event from sur- veying what are happening and how they are. For this, video is a very powerful tool now. The author started its recording a half hour later of the event when he recognized that the event had been very large scale event, and had been continuing for six hours. The major items and purposes are as follows : 1) How are there any phenomenon or failure mode which has been never observed:' In this case, they will be reported as examples in the field. 2) How adequate the models, which have been used for the design, are proven or verified through their behaviors observed? 3) How the statistics of structures and other systems are, for example, damage ratio of underground pipeline etc.? 4) How to recover structures and systems from their damaged state in a short range, hours and day, and in long a range, month and year? 5) How did or does the event give the impact to individuals and the society? 1.3 Facts on the Event Magnitude : M = 7.2 Focal Depth : H = 14 km 38 Time and Date : 05:46 , 1/17/1995 , JST Casualties : 5,500 deaths + 35,000 injures Damaged Buildings & Houses : App. 160,000 . Estimated Total Loss : $ 100 Billion Those numbers on damages are still moving 1.4 Fault, Ground Motion and Intensity The slips of related fault have been studied by seismologists, and various kind of new- facts have been being found. One of the features is high velocity ground motion. Japanese Meteological Intensity Seale exceeded VI in this event. This lias been the first event since it was denned after Fukui earthquake in 1948. The definition of Intensity VII is as follows : / = VII > 400 gal and Damage rate of buildings > 30 % There were several issues, for example, how to defined the damage rate and so on this time, but JMA decided that in the area of the center of Kobe City the intensity was VII. Following issues on their feature of ground motions which were made immeadiatly after the event, now some of them are clear, for example : How strong the effective PGA? Is the recorded maximum ground velocity reliable? Which faults did cause main shocks? The short duration of ground motion, is very unique as Japanese eartuquake. And initial several peaks were significant to cause structural damages as shown in Fig.l. and Table 1. Main shock consits of three shocks, and the waves were focused into the eastern part of the City Fig.2. Distribution of peak ground accelerations is shown in Fig.3. And their attenation curves are shown in Figs.4 and 5. Through the activities of the seismologists after the event, we feel the vecessity of the establishment of the engineering seismology for estimatimation of local ground motions, that is, micro-zoning. 1.5 Damage State of Various Kind of Structures i) Structures Highway Bridge : collapsing and sliding Building ; High-rise Building : less damage 39 Reinforced Conventional Building : large damages, three modes ; lower level collapse, one particular level collapse, and PS effect failure Steel Building : large damages, especially degraded one and brittle failure Wooden house : collapsing (and burnt) Piles ; failure by shear force mainly Pier and Embankment : side slip and subsidence Tower : less resonance-type failure in the area Tank : liquefaction effect, and sloshing in outside of Kobe area Comuter : overturn and cable failure Lifelines : various mode ii) Mechanism Single shock failure Resonance failure P-<5 effect Liquefaction Brittle Failure Mechanism of brittle failure has not been known well. However, the understanding on the phenomenon is diverged at each field as shown in Table 2. 1.6 Time-history and Damage The time history, that is, the partterns of ground motion of this event are very unique, and it has been proved by seismologists that there are exact reasons to induce such ground motions. The features of this event are quite different from other destructive earthquakes which have been recorded in Japan since 1880's instrumentally. The most serious destructive earthquakes, which were recorded in the past, are inter- plate type huge earthquakes and their epi-centers were in the ocean and their epicenter distances are usually more than 100 km. Durations of such earthquakes were over one minute in general. They induced resonance type failure to structures and this S-phase was followed by the surface wave period. Main points are as follows : i) Very short duration from initiation of ground motion to main peaks ; approximately 2 sec as shown in Fig.l and Table 1. ii) Similar wave form of acceleration to displacement : 40 This means, that the waves consist of rather simple component distribution, and it makes easy to analyse them from seismological view-points. Comparison of NS motions at Kobe Ocean Observatory to the images of Video, recorded at 10 sec ahead at the NHK office in the area shows the process of events as Table 1. The shaking tests of human on the table, demonstrate the strong effect to human body as well as to structures. The large amplitude of displacemet in a short duration may cause many unique features of this event, especially, many deaths. It should be mentioned that such a type of ground motions may be expected more in low seismicity area rather than high seismicity area, even though the probability of occurence might be low. 1.7 Response Spectrum and Particle Motions. Role of Vertical Component They have been discussed, but their direct effect has not known exactly yet. Some features are as follows : i) Dominant in longer period range, 1 ~ 1.5 sec in the response spectrum (in Fig.9 and 10) ii) Corelation of horizontal motions and vertical motions is recognized iii) Video-recordings at super-markets have a certain role for seismological study iv) Behavior of box-shape articles, over-turning, sliding and jamping As far as the second item, we feel the strong necessity of more study. 1.8 Damage of Industrial Facilities There were very serious damages of industrial facilities such as ship-builders, macline factories, habor facilities, plants, lifeline and so on. Most of them came from liquefaction rather than acceleration effect. Therefore, those were found more in areas near to the coast line compared to ordinary buildings and residential houses. 1) Effect of liquefaction and land-sliding : i) Overhead piping system above-ground deformation ii) Crane failure including largesize container crane iii) Settlement of heavy articles iv) Deformation of rails for O.H. crane and other traveling machines v) Ship-builders' yard and dock vi) Settlement of cylindrical tank vii) Lifeline, underground pipings viii) Switching station 41 2) Accerelation effect i) Railway train derailing ii) Power plant, boiler and pipings iii) Switching station and transmission line iv) Towers-type crane v) Tower for micro-wave transmission and other purpose 1.9 Damage Modes.